5 research outputs found
Prediction of the Wetting Behavior of Active and Hole-Transport Layers for Printed Flexible Electronic Devices Using Molecular Dynamics Simulations
Molecular dynamics (MD) simulations
were used to predict the wetting behavior of materials typical of
active and hole-transport layers in organic electronics by evaluating
their contact angles and adhesion energies. The active layer (AL)
here consists of a blend of poly(3-hexylthiophene) and phenyl-C<sub>61</sub>-butyric acid methyl ester (P3HT:PCBM), whereas the hole-transport
layer (HTL) consists of a blend of poly(3,4-ethylenedioxythiophene)
and poly(styrenesulfonate) (PEDOT:PSS). Simulations of the wetting
of these surfaces by multiple solvents show that formamide, glycerol,
and water droplet contact angle trends correlate with experimental
values. However, droplet simulations on surfaces are computationally
expensive and would be impractical for routine use in printed electronics
and other applications. As an alternative, contact angle measurements
can be related to adhesion energy, which can be calculated more quickly
and easily from simulations and has been shown to correlate with contact
angles. Calculations of adhesion energy for 16 different solvents
were used to rapidly predict the wetting behavior of solvents on the
AL and HTL surfaces. Among the tested solvents, pentane and hexane
exhibit low and similar adhesion energy on both of the surfaces considered.
This result suggests that among the tested solvents, pentane and hexane
exhibit strong potential as orthogonal solvent in printing electronic
materials onto HTL and AL materials. The simulation results further
show that MD can accelerate the evaluation of processing parameters
for printed electronics
Molecular Modeling of Interfaces between Hole Transport and Active Layers in Flexible Organic Electronic Devices
Molecular modeling methods are used
to understand the interfacial
properties between the hole-transport and active layers in organic
photovoltaic (OPV) devices. The hole-transport layer (HTL) consists
of a blend of poly(styrene-sulfonate) and poly(3,4-ethylenedioxythiophene)
(PEDOT:PSS), whereas the active layer (AL) consists of a blend of
poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM).
Simulation results on the HTL confirm the interpenetrating lamellar
structure with alternating PSS and PEDOT domains as observed in experiments.
In addition, interfacial results show high PCBM interactions with
the HTL, which result in PCBM migration to the HTL surface. The observed
PCBM concentration profile is discussed from the perspective of attractive
interactions, and it is shown that these interactions are governed
by the side chain of PCBM. Calculations also suggest that OPV device
performance could be improved by, for example, increasing the number
of benzene rings and backbone −CH<sub>2</sub>– groups
in the PCBM side chain, which would be expected to reduce PCBM concentration
at the HTL surface. The results yield important insights into molecular
interactions associated with the HTL and AL interfaces that contribute
to final device morphology and thus provide guidelines toward materials
design approaches for optimized device performance
РП Спецсеминар Совр. проблемы биофизики, биологии и биотех-и 2015 с печатью
Silica
nanostructures find applications in drug delivery, catalysis,
and composites, however, understanding of the surface chemistry, aqueous
interfaces, and biomolecule recognition remain difficult using current
imaging techniques and spectroscopy. A silica force field is introduced
that resolves numerous shortcomings of prior silica force fields over
the last 30 years and reduces uncertainties in computed interfacial
properties relative to experiment from several 100% to less than 5%.
In addition, a silica surface model database is introduced for the
full range of variable surface chemistry and pH (Q<sup>2</sup>, Q<sup>3</sup>, Q<sup>4</sup> environments with adjustable degree of ionization)
that have shown to determine selective molecular recognition. The
force field enables accurate computational predictions of aqueous
interfacial properties of all types of silica, which is substantiated
by extensive comparisons to experimental measurements. The parameters
are integrated into multiple force fields for broad applicability
to biomolecules, polymers, and inorganic materials (AMBER, CHARMM,
COMPASS, CVFF, PCFF, INTERFACE force fields). We also explain mechanistic
details of molecular adsorption of water vapor, as well as significant
variations in the amount and dissociation depth of superficial cations
at silica–water interfaces that correlate with ζ-potential
measurements and create a wide range of aqueous environments for adsorption
and self-assembly of complex molecules. The systematic analysis of
binding conformations and adsorption free energies of distinct peptides
to silica surfaces will be reported separately in a companion paper.
The models aid to understand and design silica nanomaterials in 3D
atomic resolution and are extendable to chemical reactions
Prediction of Specific Biomolecule Adsorption on Silica Surfaces as a Function of pH and Particle Size
Silica
nanostructures are biologically available and find wide
applications for drug delivery, catalysts, separation processes, and
composites. However, specific adsorption of biomolecules on silica
surfaces and control in biomimetic synthesis remain largely unpredictable.
In this contribution, the variability and control of peptide adsorption
on silica nanoparticle surfaces are explained as a function of pH,
particle diameter, and peptide electrostatic charge using molecular
dynamics simulations with the CHARMM-INTERFACE force field. Adsorption
free energies and specific binding residues are analyzed in molecular
detail, providing experimentally elusive, atomic-level information
on the complex dynamics of aqueous electric double layers in contact
with biological molecules. Tunable contributions to adsorption are
described in the context of specific silica surface chemistry, including
ion pairing, hydrogen bonds, hydrophobic interactions, and conformation
effects. Remarkable agreement is found for computed peptide binding
as a function of pH and particle size with respect to experimental
adsorption isotherms and ζ-potentials. Representative surface
models were built using characterization of the silica surfaces by
transmission electron microscopy (TEM), scanning electron microscopy
(SEM), Brunauer–Emmett–Teller (BET), thermalgravimetric
analysis (TGA), ζ-potential, and surface titration measurements.
The results show that the recently introduced interatomic potentials
(Emami et al. <i>Chem. Mater.</i> <b>2014</b>, <i>26</i>, 2647) enable computational screening of a limitless
number of silica interfaces to predict the binding of drugs, cell
receptors, polymers, surfactants, and gases under realistic solution
conditions at the scale of 1 to 100 nm. The highly specific binding
outcomes underline the significance of the surface chemistry, pH,
and topography
Chemistry of Aqueous Silica Nanoparticle Surfaces and the Mechanism of Selective Peptide Adsorption
Control over selective recognition of biomolecules on
inorganic nanoparticles is a major challenge for the synthesis of
new catalysts, functional carriers for therapeutics, and assembly
of renewable biobased materials. We found low sequence similarity
among sequences of peptides strongly attracted to amorphous silica
nanoparticles of various size (15–450 nm) using combinatorial
phage display methods. Characterization of the surface by acid base
titrations and zeta potential measurements revealed that the acidity
of the silica particles increased with larger particle size, corresponding
to between 5% and 20% ionization of silanol groups at pH 7. The wide
range of surface ionization results in the attraction of increasingly
basic peptides to increasingly acidic nanoparticles, along with major changes
in the aqueous interfacial layer as seen in
molecular dynamics simulation. We identified the mechanism of peptide
adsorption using binding assays, zeta potential measurements, IR spectra,
and molecular simulations of the purified peptides (without phage)
in contact with uniformly sized silica particles. Positively charged
peptides are strongly attracted to anionic silica surfaces by ion
pairing of protonated N-termini, Lys side chains, and Arg side chains
with negatively charged siloxide groups. Further, attraction of the
peptides to the surface involves hydrogen bonds between polar groups
in the peptide with silanol and siloxide groups on the silica surface,
as well as ion–dipole, dipole–dipole, and van-der-Waals
interactions. Electrostatic attraction between peptides and particle
surfaces is supported by neutralization of zeta potentials, an inverse
correlation between the required peptide concentration for measurable
adsorption and the peptide p<i>I</i>, and proximity of cationic
groups to the surface in the computation. The importance of hydrogen
bonds and polar interactions is supported by adsorption of noncationic
peptides containing Ser, His, and Asp residues, including the formation
of multilayers. We also demonstrate tuning
of interfacial interactions using mutant peptides with an excellent
correlation between adsorption measurements, zeta potentials, computed
adsorption energies, and the proposed binding mechanism. Follow-on
questions about the relation between peptide adsorption on silica
nanoparticles and mineralization of silica from peptide-stabilized
precursors are raised